to start each cat should have there own littre box avaible to them. Although I have two they tend to use eachothers its just easier to keep them cleaner longer plus some cats dont like to share, but you could always get another cat littre box (can be bought at a dollar store) if you don't have one already that is and put some of the new littre in the new box see if they take to it ? if your cats are anything like mine they wont really care about the littre and will use your littre box anyways, or you could, try mixing a little of both littres togethier.